Behat tests for forum "postwithoutthrottling" capability logic (Automate MDLQA-15)

QA testing instructions

1. Log in as a teacher and create a forum with a time period for blocking set.
2. Set a post threshold for blocking and for warning.
3. Ensure the student role does not have capability "mod/forum:postwithoutthrottling" within the forum.
4. Log in as a student and check that a warning is show on the forum post screen after reaching the number of posts specified in the forum setting 'Post threshold for warning'.
5. Keep posting until the number of posts reaches the forum setting 'Post threshold for blocking'.
6. On your last post you will have been given a time frame that you are allowed to edit this post, ensure you still can edit it and are not thrown an error about reaching the threshold.
7. Click to post another reply and ensure you are thrown the error "You have exceeded the posting threshold set for this forum".
8. Change the the "mod/forum:postwithoutthrottling" capability for the student role to Allow.
9. Check that the student can now post an unlimited number of times.

There's behat coverage for forum post threshold but not for the capability checks.

Once done, please change the affected QA parent to MDLQA-5249